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Q: How do I control floating and bottom-growing algae in my lake? – Tom in New York

A: Whether it’s floating or submerged, algae can turn a lake into a green mess in no time. It’s unsightly, it’s sometimes stinky and in extreme cases, it can cause a fish kill. The good news is that algae can be controlled no matter what time of year. It starts with controlling the population and ends with a long-term management plan.

Before we dive in, it’s important to understand the difference between algae and weeds. The term “algae” refers to a wide range of single and multi-celled organisms that live in the water and metabolize carbon dioxide into oxygen via photosynthesis, just like plants. They differ from plants or weeds in that they don’t have true leaves, roots or stems.

In lakes and ponds, the most common varieties of algae include: Green floating algae that creates a “pea soup” appearance; Chara or Stonewort, which are a bottom-growing, seaweed-looking type that can be mistaken for weeds, and string or filamentous algae, which are actually long strings of algae connected together.

Sometimes, pond and lake owners may mistake duckweed for floating algae, but if you look very closely, you’ll find that it’s actually duckweed or watermeal. Check out this blog entry to learn more about controlling this invasive weed.

Population Check

If your pond is coated in pea soup or the bottom is carpeted in Chara or string algae, you can knock back the population with a chemical herbicide like Algae Defense® . It provides quick results and it’s formulated to get a pond under control – especially during the hot summer months. Do not use if your pond or lake is stocked with koi or goldfish. If your pond has trout, check your carbonate hardness with a water hardness test kit, like the Laguna Quick Dip Multi-Test Strips, and make sure the carbonate hardness is above 50 parts per million (ppm) before using Algae Defense®.

Long-Term Strategy

Algae Defense® by Pond Logic® will solve a crisis, but to keep your pond or lake looking clean and clear, you’ll need to be proactive and develop a plan to manage the algae. The most successful approach centers on cutting off the algae’s food supply – nutrients.

Nutrients can come from a wide variety of sources, like grass clippings, twigs, trees, fish waste, yard and farm fertilizers and runoff. As these nutrients break down, they produce ammonia, which triggers the nitrogen cycle. Nitrifying bacteria surround the ammonia, turning it into nitrites and then into nitrates (nutrients) – which then feed the algae.

So, how do you reduce the nutrients in your pond?
Try these tips:

Buffer before fertilizing: To prevent inadvertently fertilizing the algae, leave a buffer area around the pond. You can also try using organic or low-phosphorus fertilizers.

Aerate, aerate, aerate: Because that muck at the bottom of the pond feeds the algae, you should prevent the buildup with proper aeration.

Reduce the muck: Use natural bacteria like MuckAway™ by Pond Logic® to breakdown up to 5-inches of organic muck per year. You can also rake your pond using a Pond & Beach Rake to remove dead vegetation, leaves and other organics that will eventually decompose on the bottom.

Reduce sunlight: Like all photosynthetic organisms, algae requires sunlight to thrive. Adding pond dye can help provide shade. If possible, consider adding some non-invasive aquatic plants to your pond. The plants, which also consume nitrates, will also be a source of competition for food.

Add beneficial bacteria: You may also consider adding some additional beneficial bacteria, like PondClear™ by Pond Logic®, to your pond or lake. The bacteria gobble through nitrates, breaking down fish waste, leaves and other organics that accumulate in the pond, naturally improving the water clarity.

That green gunk can be controlled in your pond or lake. It just takes a little planning and some proactive management. When you see the results, it’ll be worth it!

POND TALK: When was your worst algae bloom and how did you control it?

24 Responses

I Have a koi pond, 15’x15′, approx 4′ deep at the deepest level. I have 2 waterfalls,and a bog, with lots of plants, some hyacinths floating, tho not to their full potential yet. I am confused as to what to use for algae control. Have some string algae in the waterfall, the rocks have “fuzz’ on them, the water is not murky, but not pretty& clear. I once used a product that caused a big mess with breaking down the algae ( I suppose) and had to drain & clean the whole pond. now I am afraid to use anything! what do you suggest? Also, what about adding salt to my pond? I live in middle GA. Thanks, Pat

As far as your “fuzz” on the waterfall rocks are concerned, Pond Logic Oxy-Lift Defense® can be used to simply “lift” debris from waterfalls, streams, rocks, statues, birdbaths and/or anywhere debris occurs. It is great for ponds with fish & plants. Once the algae has died off, it is best to remove it from the pond if possible. When dead algae sinks to the bottom it helps feed the fire for algae.

Pond Logic Oxy-Lift Defense is sold separately and also in a DefensePac®. The Pond Logic DefensePac® provides crystal clear water in 5 simple steps with this easy-to-use all-natural pond maintenance system. Pond Logic® combines 5 of the most effective pond care products into a comprehensive maintenance package saving you time, money and unnecessary guesswork. The Pond Logic® DefensePAC® combats common pond problems such as substandard water quality, unwanted muck, built up debris and poor fish health. The DefensePAC® is available in two sizes both of which are packaged in a re-sealable bucket with complete easy-to-follow instructions.

can you guys help me, ive had the same pond for 25 years its about a 50×75 shallow on edges but 10-15ft deep at my damn and i have the floating green alge i guess it moves to one side or the other when a storm hits and was wondering what i can do to clear it up it has just started a couple of years ago and as you stated it is near farm land and my new neighbor is a farmer and his run off goes into my pond he does have gaots and such and it seems like this is when it started what can i do to correct this thanks wayne

I would begin by using an aeration system. This will increase circulation so the run off does not just build up in the pond encouraging more muck and growth. Natural bacteria will also help reduce the muck. For a short-term fix chemicals are useful, but chemical use alone will begin a never ending battle.

If a permit is required you must obtain a permit to use the product. Every state has their own regulations. Typically single homeowner, enclosed ponds, do not require permits hwere association ponds or ponds flowing to public waterways do. We always suggest contacting your local environmental agency about regulations before applying any product to the water.

Water hyacinths are great for small decorative ponds and yes they will compete for the same food source as algae and when there is enough of them, they will help prevent growth.

In a large pond such as the blog article describes above, I wouldn’t advise using water hyacinth, because the are very invasive and can take over your entire large pond or lake quite quickly and instead of an algae problem, now you have a water hyacinth problem.

I contacted our service department to see if they can help me with providing information about your pond, but in the interim could you answer a few questions for me?:

1.) How big in the pond?
2.) Do you live around farm land where fertilizer is used? Do you use fertilizer for your grass?
3.) Do you have any trees around your pond?
4.) Is there a way you can send me a picture of your pond to identify the algae. Please e-mail me at: blog@thepondguy.com

Hi Vicki – Any type of water-loving plant has the potential to become invasive in a larger unlined pond. If you really want to add plants, I would either choose tropical plants that will die off at the end of the season, or put your plants in pots to keep them contained.

I also have a roughly 1/4 acre 9′ deep pond in SW Michigan. Like yourself I’m interested in incorporating some non-invasive aquatic plants into the pond. I’ve managed to get the cattails under control and I’m making progress on the string algea. Once complete I would enjoy creating some aquatic plant life the bass and blue gill will continue to enjoy. I would enjoy sharing any ideas you may have.

Hi William – If you want quick results, you may want to check in your area for a local excavator that would be able to dredge the pond. If you are willing to wait a little longer, an aeration system, along with the use of natural bacteria, like PondClear by Pond Logic, will eat away at the muck on the bottom of the pond. In optimum conditions, this could reduce up to 5 inches of muck per year!

Thanks for writing, Ari. The main difference between PondClear and MuckAway is the way they attack the muck. PondClear will release at the surface of the water to begin working on suspended debris in the water body. MuckAway will sink right to the bottom of the pond to directly begin working on the accumulated debris on the pond’s bottom. You can use both products together. A clearer body of water will allow more light to penetrate the pond, but dye and aeration — in addition to the bacteria — will work to keep the pond clean. While the debris may stop some light penetration in the pond, the debris is also acting as a food source for algae, thus also promoting growth — not to mention taking away oxygen for your fish.

cool. but here’s a followup. if you can use pond clear and muck away – do you need to put half the amount you normally would of each? Otherwise, if it’s the same bacteria, wouldn’t you be double dosing the pond? Thanks!

Hi, I have a small piece of the Ohio Erie Canal which is about 300 feet long by 25 feet wide and is no more than 4 to 5 feet deep. I have the problem of algae and have coi and gold fish in the pond. I can’t use Algae Defense due to the fish, you stated. What can I do to reduce the algae problem? I also have few trees around the pond. Shade to reduce algae growth, so I thought, but leaves to contaminate and cause muck. Which way should I go. MuckAway would help, but will it also take care of the two types of algae I have? String and the pea soup type. Help!!!

Rick, You mentioned that this is part of the canal, is this area completely enclosed or open to the rest of the water body? If the area is open, you will need to contact your local environmental department for treatment options. If the area is completely enclosed and does not feed to surrounding areas, you can use Algae Defense if your carbonate hardness is about 50 ppm, or you can use Hydrothol 191 granular. The granular will work on the algae on the sides of the pond, but may not work as well on floating or pea soup algae. Your best bet in that case is to use natural bacteria and aeration to remove the muck in the pond and eventually the pond should begin to clear up.

my section of the old canal is completely closed in. I tried muck away. It helped some, I also have a fountain in the center of the canal to areate it. It is stocked with plenty of live fish, including catfish and carp for bottom control. Just placed some grass morel fish to see if they would eat some algae. shallow water and few trees around it makes lots of sun but also gets leaves in fall to cause build up! Love your catalogs and articles Thanks.

i have a 88000 gallon lined pond that i just drained and power washed to remove algie/muck, i re filled the pond. no fish are in the pond .
i have 2 skimmer boxes w/ pumps going to my water fall, should i run aeration in it to keep it clean?
thhanks for your help

Hi Joe – We recommend always running your aeration to help keep debris from accumulating on the bottom of the pond. Prevention is key. If you let the pond sit stagnant, then the debris will build up again; if you keep it running, you shouldn’t have to keep doing large scale clean-outs.

The waterfalls will help with some aeration, but not all. With a decorative pond around 88,000 gallons, I would assume the depth of the pond would be around 3-4′ or deeper. If this is true I would recommend an aerator to not only help aerate the pond, but also circulate. When the depth is 3-4′ in a decorative pond with a skimmer, what happens is the water that is pulled into the skimmer is always from the surface. The water on the bottom is never pulled through. With an aeration system, however, the water will be lifted from the bottom of your pond and circulated through the skimmer. Make sense?

Also, having the oxygen saturated in the water will increase aerobic bacteria activity to help keep your water clean. would suggest using a large pail of Nature’s Defense and large pail of Muck Defense to help keep down suspended organics as well as bottom muck.